Acquired Immunodeficiency Syndrome (AIDS) is one of the most significant infections to appear in recent history. This epidemic is not confined to a single segment of the population nor is its spread blocked by natural barriers or international boundaries. Millions have died in Africa and many more individuals are infected worldwide. In the United States more than 100,000 people have died and at least 1 million more are presently infected with the virus. Although there are some new drug treatments that are currently available for treating the disease, this pandemic shows no signs of abating.
AIDS is a disease of an acquired immunodeficiency syndrome in humans caused by HIV. The first description of the disease was in 1981. Its causative agent, HIV, was discovered in 1983. As of 1993 it is estimated that about 13 million people were infected with HIV worldwide and this number has increased to about 21 million in 1996. See B. Jasny, Science, 260(5112), 1219 (1993) and P. Piot, Science, 272(5270), 1855 (1996).
AIDS was first diagnosed in male homosexuals who exhibited a variety of infections of fungal (Candida albicans), protozoal (Pneumocystis carinii), and viral (Herpes zoster) origin. Many of these individuals also had an increased incidence of kaposi sarcoma and lymphoma. They had a depressed T helper/T suppressor lymphocyte cell ratio and an absence of delayed hypersensitivity responses. Collectively, these observations suggested a deficiency in cell-mediated immunity.
The causative agent in AIDS is an RNA retrovirus called the human immunodeficiency virus (HIV-1 or HIV-2). HIV possesses an envelope glycoprotein (gp120) that has a high affinity for the CD.sub.4 receptor on T helper cells and other target cells. These other target cells include bone marrow stem cells, macrophages, endothelial cells, glial cells, lymph node, dendritic cells, bowel enterochromaffm cells, cervical eptithlium and possibly Langerhans cells. However, it is the effects of HIV on T-helper cells that are the best known. The infectious process begins when the virus penetrates the body and enters the blood stream. Binding of HIV to CD.sub.4 target cells involves interaction of the external envelope glycoprotein molecule gp120 with the CD.sub.4 molecule, although other cell receptors may be involved. The virus next enters the target cell, or is internalized, through fusion of the viral envelope with the target cell membrane. Through this fusion, the virus loses its coat, and releases its RNA core and reverse transcriptase enzyme into the host cell cytoplasm.
The HIV reverse transcriptase enzyme copies the RNA message producing first a single-stranded, and then a double-stranded, DNA (circular complementary DNA). This newly formed double-stranded DNA becomes incorporated into the host chromosomal DNA once it enters the host cell nucleus. This incorporated viral DNA may remain dormant or, upon activation, will produce viral messenger RNA (mRNA). The viral MRNA codes for proteins that are important in viral replication. Glycoprotein will then envelop the RNA genome resulting in the production of infectious viral particles; completed viral particles are then released to infect other cells.
In the efforts to combat the disease several drugs have been approved by the FDA for treating this disease, including azidovudine (AZT), didanosine (dideoxyinosine, ddI), d4T, zalcitabine (dideoxycytosine, ddC), nevirapine, lamivudine (epivir, 3TC), saquinavir (Invirase), ritonavir (Norvir), indinavir (Crixivan), and delavirdine (Rescriptor). See M. I. Johnston & D. F. Hoth, Science, 260(5112), 1286-1293 (1993) and D. D. Richman, Science, 272(5270), 1886-1888 (1996).
Many of the drugs currently approved for AIDS treatment utilize inhibition of viral proliferation and are viral reverse transcriptase inhibitors or viral protease inhibitors. Other protease inhibitors, such as nelfinavir and improved saquinavir, are either in development or close to approval. An AIDS vaccine (Salk's vaccine) has been tested and several proteins which are chemokines from CD8 have been discovered to act as HIV suppressors.
In addition to the synthetic nucleoside analogs, proteins, and antibodies noted above, several plants and substances derived from plants have been found to have in vitro anti-HIV activity, such as Lonicera japonica and Prunella vulgaris, and glycyrrhizin from Glycyrrhiza radix. See R. S. Chang & H. W. Yeung, Antiviral Research, 9, 163-175 (1988) and M. Ito, et al., Antiviral Research, 7, 127-137 (1987).
Despite all of the available pharmaceuticals for the treatment of HIV, there is still no cure for the deadly disease. HIV viruses continue to mutate and become resistant to existing drugs such as the reverse transcriptase inhibitors and protease inhibitors. Recently, a therapy of using two (2) or three (3) anti-HIV drugs in combination has been found effective in significantly lowering the HIV loads in AIDS patients. The results have been promising, however the virus continues to develop resistance to the drugs and the long-term outcome (survival and cure rates) is still unknown. Thus, the medical communities throughout the world continue to search for drugs that can prevent HIV infections, treat HIV carriers to prevent them from progressing to full-blown deadly AIDS, and treat the AIDS patient.
As noted above, many AIDS suffers are also afflicted with Kaposi's sarcoma. In fact, Kaposi's sarcoma is the most common neoplasm occurring in persons with acquired immunodeficiency syndrome (AIDS). Approximately 15-20% of AIDS patients develop this neoplasm which rarely occurs in non-AIDS infected individuals. Epidemiologic evidence suggests that AIDS-associated Kaposi's sarcoma(AIDS-KS) may have an infectious etiology. Kaposi's sarcoma-associated herpesvirus (KSHV/HHV8) is a new human herpesvirus found in all KS lesions and considered the infectious agent. Gay and bisexual AIDS patients are approximately twenty times more likely than hemophiliac AIDS patients to develop Kaposi's sarcoma, and Kaposi's sarcoma may be associated with specific sexual practices among gay men with AIDS. Kaposi's sarcoma is uncommon among adult AIDS patients infected through heterosexual or parenteral HIV transmission, or among pediatric AIDS parenteral HIV transmission, or among pediatric AIDS patients infected through vertical HIV transmission. Agents previously suspected of causing Kaposi's sarcoma include cytomegalovirus, hepatitis B virus, human papillomavirus, Epstein-Barr virus, human herpesvirus 6, human immunodeficiency virus (HIV), and Mycoplasma penetrans. Non-infectious environmental agents, such as nitrite inhalants, also have been proposed to play a role in Kaposi's sarcoma tumorigenesis. Extensive investigations, however, have not demonstrated an etiologic association between any of these agents and AIDS-KS.
Herpesviruses, such as Kaposi's sarcoma-associated herpesvirus (KSHV), are a family of large double stranded DNA-containing viruses many members of which are important human pathogens. A ubiquitous property of the herpesviruses is their capacity to cause both acute lytic(productive) and latent infections in the human host, each of which is characterized by marked differences in viral transcription, DNA replication and in DNA structure.
Kaposi's sarcoma-associated herpesvirus (KSHV), or human herpesvirus 8 (HHV 8), has been detected in all four clinical forms of Kaposi's sarcoma: AIDS related (epidemic), European (sporadic), Transplant-associated (iatrogenic) and African (endemic). While this virus is generally absent from normal control tissues, it is present in the vast majority of AIDS- as well as non-AIDS-related KS lesions, suggesting that it is not simply an opportunistic infection in HIV-infected patients. See Chang Yet al, Identification of herpesvirus-like DNA sequences in AIDS-associated Kaposi's sarcoma. Science 266:1865-1869, 1994, Moore P S, Chang Y: Detection of herpesvirus-like DNA sequences in Kaposi's sarcoma lesions from persons with and without HIV infection. N Engl J Med 332:1181-1185, 1995, Dupin N, et al, Herpesvirus-like DNA sequences in patients with Mediterranean Kaposi's sarcoma. Lancet 345:761-762, 1995.
Furthermore, KSHV is consistently present in a specific type of non-Hodgkin's lymphoma (NHL), frequently, not exclusively, occurring in patients with AIDS, namely the primary effusion lymphomas (PELs), also called body cavity-based lymphomas (BCBLs). See Cesarman E, et al, Kaposi's Sarcoma-associated Herpesvirus-like DNA sequences in AIDS-related body cavity-based lymphomas. N Eng J Med 332:1186-1191, 1995, Nador R G, et al, Herpes-like DNA sequences in a body-cavity-based lymphoma in an HIV-negative patient (Letter to the Editor). N Engl J Med 333:943, 1995.
KSHV is also present in a significant proportion of cases of multicentric Castleman's disease, a poorly understood disorder characterized by generalized lymphadenopathies and immune disregulation. See Soulier J, et al Kaposi's sarcoma-associated herpesvirus-like DNA sequences in multicentric Castleman's disease. Blood 86:1275-1280, 1995. Recently, the presence of KSHV in bone marrow dendritic cells of all patients with multiple myeloma has been reported. See Rettig M B, et al Kaposi's sarcoma-associated herpesvirus infection of bone marrow dendritic cells from multiple myeloma patients. Science 276:1851-1854, 1997. While this last finding remains controversial, it raises the possibility of involvement of KSHV in a very frequent type of cancer affecting non-immunosuppressed individuals.
KSHV, as all other herpesvirus, has a litic and a latent cycle. Only a few antiviral agents have been used for activity against KSHV. Several studies determined the effect of acyclovir, gancyclovir, foscarnet and cidofovir in their ability to prevent lytic replication of KSHV in response to phorbol ester (TPA) induction. See Kedes D H, D. G: Sensitivity of Kaposi's sarcoma-associated herpesvirus replication to antiviral drugs. J Clin Invest 99:2082-2086, 1997, Medveczky M M, et al In vitro antiviral drug sensitivity of the Kaposi's sarcoma-associated herpesvirus. Aids 11:1327-1332, 1997, Costagliola D, et al Can anitviral agents decrease the occurrence of Kaposi's sarcoma? Clinical Epidemiology Group from Centres d'Information et de SOins de l'Immunodeficience Humaine (letter). Lancet 346:578, 1995, and Flore O, et al, Effect of DNA synthesis inhibitors on Kaposi's sarcoma-associated herpesvirus cyclin and major capsid protein gene expression. AIDS Res Hum Retroviruses vol. 13, 14: 1229-1233, 1997.
These drugs target only the litic viral replication, the latent viral infection cannot be controlled. Long-term treatment with these agents may be beneficial after the malignancy has developed, but is not resolutive. Once the treatment is interrupted the clinical reactivation can recur since the virus cannot be completely eliminated as the viral latent cycle is not affected. Accordingly, there still remains a need for a drug which inhibits the latent cycle of KSHV/HHV8 and is free from deleterious side effects.